Dequalinium (C10-DECA)(quinolinium, 1,1′-(1,10-decanediyl)bis(4-amino-2-methyl diiodide) (see 1c in Table 1) was sold for over thirty years as an antimicrobial agent and the active ingredient in mouthwash and other topical formulations. Since 1987, C10-DECA has been studied as a potent antitumor agent that recognizes several isoforms (e.g., α, β1) of protein kinase C (PKC) in vitro (IC50=10-14 μM) and in cells1,2. This monomeric serine/threonine protein kinase consists of a family of eleven structurally-related isoforms and is well-known for its role in cellular signaling pathways that govern normal cell growth and differentiation3. Because of its function in tumor formation and metastasis, PKC continues to attract interest as a target for new chemotherapeutic agents.
A potent anti-tumor agent in several animal models4, C10-DECA is accumulated by transformed cells in culture to a greater extent than by non-transformed cells5,6. C10-DECA inhibits PKC activity in vitro and in cells at low micromolar concentrations1,2 by interfering with binding sites in both the regulatory1,2 and catalytic domains2,7. Inhibition of catalytic activity involves a site or sites located in the catalytic domain of PKCα2,7, but an additional C10-DECA recognition site in the regulatory domain involving the RACK-1 (Receptor for Activated C-Kinase) binding domain has also been demonstrated1.
A series of PKCα mutants that represented progressive truncation from the amino-terminus (regulatory domain) have been tested for sensitivity to inhibition to C10-DECA in order to examine the importance of the regulatory domain to C10-DECA-mediated inhibition of catalytic activity. That analysis revealed that the sites of interaction in the regulatory and catalytic domains are independent of each other, and that the site at which C10-DECA produces inhibition of catalytic activity lies exclusively in the catalytic domain7. One of the inventors (SAR) has tested structurally homologous protein kinases for inhibition, and found them to be insensitive to micromolar C10-DECA concentrations. The homologous protein kinases included the cAMP-dependent protein kinase (PKA), the calmodulin-dependent myosin light chain kinase, and the pp60src tyrosine protein kinase. The finding that PKA is insensitive to DECA at high micromolar concentrations is compelling in view of the close sequence homology of the PKC and PKA catalytic domains. With respect to these other protein kinases, the key site recognized by C10-DECA is apparently unique to PKC. Additional cellular targets of C10-DECA have been reported, however, namely the mitochondrial F1-ATPase8, calmodulin-dependent phosphodiesterase9, and the calcium-activated K+-channel10.
An unusual property of C10-DECA is that it can be photolyzed with longwave UV light, which converts it into an irreversible inhibitor of PKC activity1. Photoreactivity was previously employed to demonstrate inactivation of recombinant PKCα activity in vitro and PKCα translocation in cells1.
There is a need, however, to optimize the ability of C10-DECA to inhibit the activity of an isoform of protein kinase C, especially protein kinase Cα. There is a particular need to optimize the beneficial manifestations of inhibiting the activity of an isoform of protein kinase C, such as inhibiting the motility of pre-cancerous cells, inhibiting the growth of pre-cancerous or malignant cells, and inhibiting the metastasis of malignant cells.